linux-kernel - Re: [fuse-devel] [PATCH] fuse: break infinite loop in fuse_fill_write

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Date:	Sat, 3 Oct 2015 04:58:46 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Maxim Patlasov <mpatlasov@...allels.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	fuse-devel@...ts.sourceforge.net,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Miklos Szeredi <miklos@...redi.hu>
Subject: Re: [fuse-devel] [PATCH] fuse: break infinite loop in fuse_fill_write_pages()

On Sat, Oct 3, 2015 at 1:04 AM, Andrew Morton <akpm@...ux-foundation.org> wrote:
> On Fri, 2 Oct 2015 12:27:45 -0700 Maxim Patlasov <mpatlasov@...allels.com> wrote:
>
>> On 10/02/2015 04:21 AM, Konstantin Khlebnikov wrote:
>> > Bump. Add more peopple in CC.
>> >
>> > On Mon, Sep 21, 2015 at 1:02 PM, Roman Gushchin <klamm@...dex-team.ru> wrote:
>> >> I got a report about unkillable task eating CPU. Thge further
>> >> investigation shows, that the problem is in the fuse_fill_write_pages()
>> >> function. If iov's first segment has zero length, we get an infinite
>> >> loop, because we never reach iov_iter_advance() call.
>>
>> iov_iter_copy_from_user_atomic() eventually calls iterate_iovec(). The
>> latter silently consumes zero-length iov. So I don't think "iov's first
>> segment has zero length" can cause infinite loop.
>
> I'm suspecting it got stuck because local variable `bytes' is zero, so
> the code does `goto again' repeatedly.
>
> Or maybe not.   A more complete description of the bug would help.

I suspect here is the same scenario like in 124d3b7041f:
Zero-length segmend is followed by segment with invalid address:
iov_iter_fault_in_readable() checks only first segment (zero-length)
iov_iter_copy_from_user_atomic() skips it, fails at second and
returns zero -> goto again without skipping zero-length segment.

Patch calls iov_iter_advance() before goto again: we'll skip zero-length
segment at second iteraction and iov_iter_fault_in_readable() will detect
invalid address.
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